1. Field of the Invention
The present invention relates to a recording medium transport device that nips and transports a recording medium, and an image forming apparatus including the recording medium transport device and a printing part.
2. Background Art
Conventionally, in an apparatus for forming an image by causing ink jetted from a nozzle of an ink-jet recording head to adhere to a recording medium, the recording medium (hereinafter referred to as a sheet) is made to intermittently move (step feeding) in a sub-scanning direction every specified length, and during the stop of the intermittent movement, a carriage on which the recording head is mounted is moved in a main scanning direction, and an image is formed in every specified area.
In that case, an upstream side transport roller pair at a transport upstream side (hereinafter simply referred to as an upstream side) in a sheet transport direction (sub-scanning direction) and a downstream side transport roller pair at a transport downstream side (hereinafter simply referred to as a downstream side) are disposed at both sides of the recording head, the sheet is nipped by both the roller pairs, and both the roller pairs are intermittently driven to move the sheet in the sub-scanning direction.
Incidentally, it is known from JP-A-10-77136, etc., that in order to control a state in which the leading end of the sheet is nipped by the upstream side transport roller pair and a timing when the recording head starts a printing operation to the sheet, a registration detection sensor (sheet detection sensor) for detecting the leading end of the sheet and a registration detection lever (sheet detection lever) are provided at a slightly upstream side of the upstream side transport roller pair.
In the structure according to JP-A-10-77136, a pipe shaft long in the main scanning direction is rotatably supported at a halfway part of a pinch roller holder for supporting a pinch roller pressed to a drive roller of the upstream side transport roller pair, and a front lever piece is fixed to one side (right end part) of the pipe shaft. The front lever piece is made to face on a lower place from a rectangular hole bored in the right end part of the pinch roller holder, and the tip of the front lever piece is positioned on a side of a nipped part of the sheet by the upstream side transport roller pair. Then, an upper end of a rear lever piece provided to be coupled with a left end part of the pipe shaft is urged by a torsion spring so that it enters the sheet detection sensor composed of a photo interrupter.
On the other hand, it is known from JP-A-7-76140, etc., that a width detection sensor for detecting a sheet width is provided, and a printing work area is controlled correspondingly to the sheet width in a direction orthogonal to a sheet transport direction.
In JP-A-7-76140, plural sheet detection levels are disposed in the main scanning direction in, for example, ascending order of sheet width and at respective partition places of a postcard size, a B5 size, a LTR (letter) size, and an A4 size, and the plural sheet width detection levers are disposed while their phases are shifted from each other in the rotation direction. In addition to those, one second switch lever is disposed at a downstream side of detection positions of the plural sheet width detection levers, and a difference in time between detection of a sheet by the sheet width detection lever and detection of the sheet by the second switch lever is judged to detect the sheet size.
In the mechanical sensor such as the registration sensor (the sheet detection lever, the registration detection lever) and the sheet width detection lever in JP-A-10-77136 and JP-A-7-76140 as the prior art, every lever is spring urged in a direction of blocking the sheet transport path in a state where the sheet is not detected.